The present invention relates generally to ultrasonic diagnostic systems, and, more particularly, to a system and method for visualizing scene shift in an ultrasound scan sequence.
Ultrasonic transducers and imaging systems have been available for quite some time and are particularly useful for non-invasive medical diagnostic imaging. Ultrasonic transducers are typically formed of either piezoelectric elements or of micro-machined ultrasonic transducer (MUT) elements. When used in transmit mode, the transducer elements are excited by an electrical pulse and in response, emit ultrasonic energy. When used in receive mode, acoustic energy impinging on the transducer elements is converted to a receive signal and delivered to processing circuitry associated with the transducer.
The transducer is typically connected to an ultrasound imaging system that includes processing electronics, one or more input devices and a suitable display on which the ultrasound image is viewed. The processing electronics typically include a transmit beamformer that is responsible for developing an appropriate transmit pulse for each transducer element, and a receive beamformer that is responsible for processing the receive signal received from each transducer element.
An ultrasonic transducer is typically combined with associated electronics in a housing. The assembly is typically referred to as an ultrasonic probe. Typically, ultrasonic probes are classified as either one-dimensional (1D) probes having a single element wide array of elements, or two-dimensional (2D) probes having a multiple element wide array. Furthermore, a probe referred to as a xe2x80x9cbi-planexe2x80x9d probe includes two orthogonally positioned 1D arrays that may or may not intersect. A relatively new 2D probe, referred to as a xe2x80x9cmatrix probexe2x80x9d includes transducer elements arranged in two dimensions where each element is individually controllable, resulting in an ultrasound probe the scan lines of which can be electronically steered in two dimensions. Each dimension of a matrix probe can be thought of as a stack of contiguous linear arrays.
A matrix probe can comprise either a xe2x80x9cfully sampledxe2x80x9d or a xe2x80x9csparsely sampledxe2x80x9d aperture. In a fully sampled aperture, every transducer element is individually addressable and controllable, and all elements are contiguous. In a sparsely sampled aperture, a subset of the physical set of transducer elements is addressed and controlled, or equivalently, there is a pattern of physical gaps between some elements such that they are not all contiguous. Sparsely sampled 2D arrays allow for fewer system connections (fewer channels) while still achieving distribution of the acoustic elements in two dimensions. However, a significant drawback of sparse 2D arrays is the loss of ability to control scan beam shape.
Regardless of the type of transducer probe, many medical ultrasound imaging techniques require that successive images, referred to as successive frames, of the same part of the anatomy be taken to view the selected piece of anatomy over a period of time. One examples of such a medical ultrasound imaging technique that develops successive ultrasound images over a period of time is the measurement of myocardial perfusion.
In a myocardial perfusion measurement, successive ultrasound scans of, for example, the left ventricle of a heart are triggered by the patient""s electro-cardiogram (ECG) signal. One or more scans may be recorded per heartbeat, each at a specified delay from the ECG signal, so as to capture the same moment of the heartbeat cycle in a plurality of successive ultrasound scans, or frames. In such a measurement, the heart is imaged while an ultrasound contrast agent, such as a harmless solution of gas-filled microspheres, is introduced intravenously into the patient""s blood stream. The contrast agent circulates through the patient and eventually into the heart.
Over a sequence of ECG triggered scan images, the volume of the contrast agent entering the myocardium is recorded by the resulting echo intensity in the sequence of ultrasound images. This sequence of images is used to analyze and measure the flow of myocardial perfusion from one image to the next.
One of the drawbacks of conventional ultrasound image processing systems is that the piece of the anatomy (i.e., the heart in a myocardial perfusion measurement) tends to move, over time, with respect to the transducer probe. This condition is referred to as xe2x80x9cscene shiftxe2x80x9d over successive ultrasound images. Because the heart is in a different position during successive frames, the resulting sequence of images cannot be used to precisely determine the myocardial perfusion. This is so because, to facilitate automatic calculation of the change in intensity of the heart over time, it is necessary to align the ultrasound image data for each frame. In this manner, filtering and subtraction algorithms, for example, may operate without the detrimental effect of scene shift, which would otherwise cause apparent changes of contrast agent intensity.
One manner of addressing scene shift is to require that the patient hold their breath during successive ultrasound frames. Another manner of addressing this is for the ultrasound operator to be highly skilled in the placement and control of the ultrasound probe during successive frames. Unfortunately, both of these solutions leave much to be desired, as repeatable results are difficult to obtain.
Another manner of addressing scene shift is to employ an image alignment algorithm. Unfortunately, an image alignment algorithm is costly to implement and consumes valuable processing resources.
Another example of a medical ultrasound imaging technique that develops successive ultrasound images over a period of time is three dimensional (3D) ultrasound imaging. Modern 3D-ultrasound imaging systems generate planar scans at varying angles to the face of the transducer probe. The scan planes, referred to as slices, comprising a number of individual scan lines, are used to interrogate a volume. 3D imaging is also useful for collecting successive images, but is resource inefficient in that a large number of scan planes must be collected to interrogate a volume.
The frame size in 3D imaging is typically limited by the amount of time consumed in generating and collecting ultrasound echoes from each scan lime. The total time is referred to as xe2x80x9cframe time.xe2x80x9d The reciprocal of frame time is xe2x80x9cframe rate.xe2x80x9d The frame rate should generally be above 15 Hz for most cardiac scanning applications. Unfortunately, a 15 Hz frame rate limits the size of the volume that can be scanned, thereby making it difficult to image the entire heart.
One solution to this acoustic limitation is to collect xe2x80x9csub-volumexe2x80x9d scans over multiple heartbeats. Each of the sub-volume scans can be triggered by the above-mentioned ECG signal, and can be taken at an optimal frame rate, but is positioned at a sequential angle such that the edges of the sub-volume scans are adjacent. An image processing system associated with the ultrasound imaging system concatenates the sub-volume scans into a larger, complete volume scan.
Unfortunately, this scanning methodology also suffers from the detrimental effects of scene shift. The relative positions of the sub-volumes must be true to the positions assumed when the ultrasound energy is shifted to the sequential angles for each successive sub-volume scan. To compensate for this, as mentioned above, the patient is frequently asked to hold their breath during successive scans.
Therefore, it would be desirable to have an ultrasound imaging system capable of visualizing scene shift and determining whether the imaging subject has been displaced from one image to the next.
Embodiments of the invention include a system and method for visualizing scene shift in an ultrasound scan. In one embodiment the invention is a system for real-time visualization of scene shift in an ultrasound scan, comprising an ultrasound receiver for developing a first ultrasound image and a second ultrasound image, border formation software for determining a first border corresponding to the first ultrasound image and a second border corresponding to the second ultrasound image, and image misalignment detection software for overlaying the first border on the second border to determine whether the first border aligns with the second border.
Other systems, methods, computer readable media, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.